Implantable cardiac devices are commonly used for treating patients with heart arrhythmias. These devices include well-known pacemakers and implantable cardioverter-defibrillators (ICD's). In general, these devices include a lead that is adapted to be implanted in the body of the patient so as to be positioned adjacent the heart and a control unit that is also adapted to be implanted within the patient which is connected to the lead so as to deliver electrical impulses to the heart via the lead. Pacemaker devices can be very sophisticated and include sensors and processing capabilities so that pacing is provided only when needed.
In pacing applications, the lead is typically implanted within the chambers of the heart so as to be positioned adjacent the walls of the right atrium or the right ventricle. In many typical pacemakers, the leads are implanted so that the lead is positioned within the right atrium and the right ventricle chambers so that a pacing pulse can be delivered directly to cardiac cells of these chambers to induce a paced response of the heart.
There are several different types of leads that are currently in common use in pacing applications. One very common lead is a bipolar lead which includes a pacing electrode and a sensing electrode. The pacing pulse is delivered to the cardiac cells by the pacing electrode and the sensing electrode serves as the return path for the pacing pulse. Typically, the sensing electrode is also configured so as to monitor intrinsic heart activity and provide a signal indicative thereof to the control unit. Other types of leads include unipolar leads which have a single electrode for delivering stimulation pulses to the heart and an indifferent electrode, such as the casing of the control unit, serves as the return electrode for the stimulation pulses.
As discussed above, pacemaker electrodes are typically implanted within the right atrium and right ventricle. The right atrium and the right ventricle generally provide blood circulation to the pulmonary system, i.e., circulation to the heart itself. The left atrium and the left ventricle provide circulation to the rest of the body's circulatory system including the major organs of the body such as the brain. Typically, implantation within the right atrium and the right ventricle has been preferred to implanting leads within the left atrium and the left ventricle has generally been thought to be too invasive of a procedure and to pose undesirable risks of complications to the flow of blood in the circulatory system. However, as the blood in the circulatory system is primarily pumped by the left atrium and the left ventricle, pacing the right atrium and the right ventricle may not always provide optimum results in ensuring adequate circulation of blood in the circulatory system. In some circumstances, right atrium and right ventricle pacing does not ensure that the major organs of the circulatory system, including the brain, receive adequate blood flow.
Consequently, there has been a desire to develop techniques for directly pacing the left atrium and the left ventricle. One such technique involves the implantation of leads through the right atrium into the coronary sinus region. As used herein, the phrase "coronary sinus region" refers to the coronary sinus vein, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other cardiac vein accessible by the coronary sinus.
The coronary sinus is a vein in the coronary circulatory system that is typically located in the heart so as to be proximal the outer walls of the left atrium and left ventricle of the heart. The coronary sinus vein has an opening in the right atrium that is accessible for lead implantation. By implanting a lead within the coronary sinus region and then positioning the lead so that the lead electrodes are adjacent the left atrium or the left ventricle, it is thought that stimulation pulses can be provided to the left atrium or the left ventricle. This allows these chambers to be paced without the complications associated with directly implanting leads in these chambers.
However, there are several problems which have limited the utility of left atrium, left ventricle pacing using leads implanted within the coronary sinus region. Currently contemplated techniques for pacing the left side of the heart involve implanting a dedicated atrial lead and electrode or a dedicated ventricular lead and electrode. However, current lead designs do not facilitate implanting leads to pace both the left atrium and the left ventricle. As the leads are to be implanted within the coronary sinus region and other pulmonary veins, it is often not possible to effectively position multiple sets of leads within the confined spaces of these veins and still allow for adequate blood flow within these veins.
One possible solution to this problem is to use a single-pass lead that incorporates both the ventricular pacing and sense electrodes and separate atrial pacing and sense electrodes. The atrial and ventricular electrodes are spaced apart a set distance so that each pair of electrodes can be positioned in the coronary veins where the electrode pairs could pace the left atrium or left ventricle, respectively. However, a single lead incorporating both left atrial pacing electrodes and left ventricular pacing electrodes that are separated by a fixed distance may not result in the atrial or ventricular electrodes being positioned in the location that is best adapted for delivery of stimulation pulses to the heart of a given patient. This may be the result of either different patients having different sized hearts or the response of the hearts of different patients to stimulation pulses varying depending upon where the pacing pulse is delivered.
Moreover, implanting leads within the coronary sinus region can be a time-consuming and invasive procedure. It may not be possible for the implanting physician to be able to precisely locate both the atrial electrodes and the ventricular electrodes in the coronary venous system to provide the optimum pacing for the patient in an efficient manner.
This is especially true for leads having atrial electrodes and ventricular electrodes spaced apart a set distance. Positioning the ventricular electrode in the optimum position for pacing the left ventricle may result in the atrial electrode being positioned where it is unable to provide effective pacing for the left atrium. Moreover, the implanting physician will not generally be able to ascertain the best location for each of the electrodes until the lead is implanted. Hence, the implanting physician may have to repeatedly implant leads and then remove and replace the implanted lead with a lead that has a different separation between the atrial electrodes and the ventricular electrodes. This process may significantly increase the length and invasiveness of the implantation procedure.
Hence, there is a need for a pacing lead system that is adapted to be efficiently implanted within the coronary veins so as to be able to deliver stimulation pulses to both the left atrium and the left ventricle of the patient's heart. To this end, there is a need for a lead which will allow the treating physician to implant a lead that occupies a limited amount of space within the coronary veins, but allows the implanting physician the flexibility of being able to locate the electrodes within the coronary venous system so as to optimize the delivery of stimulation pulses to the patient.